This invention pertains to devices for measuring displacement, and, in particular, to an improved miniature differential variable reluctance transducer assembly for use in environments where a temperature gradient may exist, and for use in hard to reach areas.
There have been a number of attempts to develop highly accurate miniaturized sensors to be used by industry to measure displacement, elongation, and strain. Examples of these type of devices include U.S. Pat. No. 4,813,435 issued to Steven W. Arms on Mar. 21, 1989, based on Hall Effect sensors. Other attempts in this area include the U.S. patents issued to Robert W. Redlich, U.S. Pat. No. 4,667,158 issued on May 19, 1987 and to Alec H. Seilly, U.S. Pat. No. 4,350,954 issued on September 1982. However, these devices do not describe a method for removing errors in measurement that may be caused by temperature gradients across the transducer assembly.
The effect of temperature on inductive transducers limits their overall absolute accuracy. Inductive transducers are often designed so the measurement is made using a differential pair of coils. In this manner, the effect of temperature can theoretically be canceled, since the output signal is the difference between the output of two coils, and temperature changes that both coils experience equally is theoretically subtracted out. However, if one coil experiences a different temperature environment than the other coil, a signal proportional to the temperature gradient between the two coils will appear at the circuit output, significantly reducing absolute accuracy.
Typically the inductance is measured by using an AC excitation to drive the inductive AC bridge, and a synchronous demodulator (or other rectification means) to convert the AC signal into a DC output proportional to the physical signal of interest. The problem with this method is that it also measures any changes in DC resistance of the coil. The DC resistance of the coil is proportional to temperature, and any temperature gradients between the coils will cause an error in the measurement.
Workers in the measurement sciences field have described methods of improving measurement accuracy in the face of temperature influences; one such technique measures transducer temperature using thermoelectric voltages (Anderson, U.S. Pat. No. 5,481,199, issued on Jan. 2, 1996). However, Anderson's method requires the use of thermocouple conductors rather than the conventional copper wire used in most inductive coil assemblies. Shozo & Shinzi (Japanese Patent no. 09145495, issued on Jun. 1, 1997) described a temperature correcting device for magnetostrictive sensors which relies on a temperature sensitive diode with positive temperature characteristic to compensate for errors in the magnetostrictive sensor which has a negative temperature characteristic. These techniques rely on the addition of specialized temperature sensing materials or devices in order to achieve compensation.
In the field of flaw or crack detection it is well known that coils may be employed to induce magnetic fields in the material or structure to be tested (target). The imposed magnetic fields induce eddy currents in the target, which results in a change in impedance in the interrogation coil(s), and which may be modulated by the presence of a flaw. These methods may also be employed to construct non-contacting proximity and displacement sensors, which typically employ one or more coils and a conductive or ferrous target. Sugiyama et al. (European Patent no. 0 181 512 B1, issued Aug. 21, 1991) describe a technique for varying the depth of penetration of eddy currents in a target material by controlling both the AC excitation frequency and DC magnetic field intensity. However, Sugiyama et al. do not describe a method for compensation of thermal errors, and furthermore, they utilized separate terminals for connection of the AC and DC excitation sources to the interrogation probe.
It is the object of this invention to teach an elegant circuit for use with inductive sensors which avoids the disadvantages of and limitations of previous systems, and addresses the needs of linear position sensing in a temperature gradient environment. This invention describes a novel circuit which compensates for the effect of temperature gradients on inductive displacement transducers, but may also be applied to other types of reactive sensors. Unlike previous methods, no special temperature detecting devices, materials, or additional terminals are required in order to perform the compensation; this feature reduces system complexity, and therefore, lowers system cost.
The benefit of this circuit is especially important for miniature sensors, which exhibit high DC resistance relative to the reactive (AC resistance) component of the sensor impedance, and therefore, are more sensitive to temperature fluctuations and gradients. In addition, a method for deriving the absolute temperature of the sensor is described. This allows the inductive sensor to also serve as a temperature sensor. This signal may provide for further signal compensation or control functions without the requirement of additional temperature sensing elements. The invention described herein has applications in improved accuracy linear displacement sensing, non-contact position and proximity sensing, and eddy current sensing. Furthermore, the novel temperature compensation circuit may be combined with an inductive (or reactive) displacement sensor and appropriate structural spring element (or structure), to realize improved force, torque and acceleration transducers.